1. Field of the Invention
The invention relates to differential devices, and more particularly to driveline systems for vehicles providing variable torque transfer. More specifically, the invention relates to torque transfer using a ball-ramp-actuated friction clutch. In another of its aspects, the invention relates to control of the ball-ramp actuator by a particle clutch.
2. Description of the Related Art
In many of the vehicles being produced today, the basic vehicle platform uses front wheel drive. However, in many such vehicles, especially in vans and sport utility vehicles, it is desirable to provide four-wheel-drive, at least under certain operating conditions.
Typically, the various arrangements for achieving part-time four-wheel-drive have been complex and expensive, sometimes requiring some sort of control scheme to achieve the four wheel drive (or rear wheel drive) in response to certain predetermined operating conditions.
In certain vehicle applications, it is acceptable to provide only front wheel drive under most operating conditions. Thus, rear wheel drive is required only when the front drive wheels are slipping (e.g., under poor traction conditions). However, the prior art has not provided a suitable device wherein only front wheel drive is provided, with no substantial torque being transmitted to the rear wheels, until the front wheels begin to slip. For example, U.S. Pat. No. 4,562,897 discloses a viscous clutch in the driveline system between the front transaxle and the rear wheels. However, the system requires a selector arrangement to choose among transmitting torque through the viscous coupling, disconnecting the viscous coupling, or bypassing it.
Implementation of such a selector arrangement adds substantially to the overall complexity and cost of the driveline system, and as is well known to those skilled in the art, component cost is a major factor in determining whether or not a particular system or component will be utilized on a vehicle.
It is known from U.S. Pat. No. 5,070,975 and from European Application EP 0 314 420 to use, as a center differential in a four-wheel driveline, a viscous-actuated ball-ramp-type friction clutch to transmit torque to the rear axles when there is a speed difference between the front and rear wheels. It is believed that the torque transmitting capability of the device of the cited patent and application would be somewhat limited by the fact that one of the ball-ramp plates comprises the input to the viscous coupling. Despite the limited torque capability the device of the cited patent would most likely be substantially more expensive than would be acceptable for typical vehicle applications. One of the reasons for the excessive cost of the device of the cited patent is the use of a fairly conventional type of cast housing, having several different machined internal diameters, internal snap ring and O-ring grooves, and a set of internal splines.
An improvement to the aforementioned differentials is disclosed in U.S. Pat. No. 5,967,276, commonly owned with the instant application, wherein a ball-ramp-actuated clutch is contained in a stamped housing, reducing the cost of manufacture, and the ball-ramp actuator is itself actuated by a sealed viscous coupling, reducing the complexity of control and the need for additional control systems. The differential disclosed therein, therefore, presents a sealed unit displaying response characteristics that are fixed so long as the unit remains sealed and in good working order. An operator cannot adjust the torque transfer characteristics within the differential.
It would be advantageous, therefore, to provide a differential device that is effective and durable, and simple and cost-effective to manufacture, as the device disclosed in U.S. Pat. No. 5,967,276, yet with the flexibility of being responsive to operator control. It would further be advantageous to provide a differential device capable of displaying variable torque transfer characteristics, as may be required for differing conditions within a single application, or as may be required to use a single differential design in multiple applications without the need for substantial modification.
The invention is directed to a particle clutch having an input coupling and an output coupling. The input and output couplings each include surfaces that are oriented parallel one to the other. These surfaces are separated by a narrow gap that is filled with a magnetically-reactive medium whose properties change when exposed to a magnetic field. The medium can be in the form of fine magnetically-reactive particles, or can be a magnetorheological fluid. The ability of the medium to transfer force between the surfaces of the input and output couplings is controllable and predictable as a function of the strength of the magnetic field. The magnetic field can come from an electromagnet, a permanent magnet, or any combination of known magnets.
The invention is further directed to a differential device using a particle clutch for controlling a friction clutch. A preferred embodiment of the invention includes a friction clutch that is actuated by a ball-ramp assembly. The ball-ramp assembly includes two plates with camming elements such as balls bearing against the facing surfaces of the plates. The output coupling of the particle clutch is connected to one of the plates. When the output coupling is acted upon by the input coupling through the viscous shear of the fluid, it acts to cause the ball-ramp actuator to separate the plates and thereby engage the friction clutch.
The differential device further includes a housing having at least one internal groove running its length, and at least a first friction disc with a projection corresponding to the groove. The input coupling member of the particle clutch also includes a projection corresponding to the groove. The housing also includes an inturned shoulder at one end to retain the particle clutch axially against the ball-ramp-actuator and an outwardly extending shoulder adjacent the friction clutch. The housing accepts an endcap, within the outwardly extending shoulder, that is engaged with additional friction discs. The endcap and additional friction discs are held axially in the housing by a second inturned shoulder, but are free to rotate within the housing. When the friction clutch is engaged, the first friction disc and the additional friction discs are forced together. This causes the additional friction discs, and the endcap, to then also be rotationally connected to the housing.
In a preferred embodiment, the housing is a unitary, stamped member having generally constant wall thickness, with a plurality of axially extending grooves. The first friction disc and the input coupling member can each include multiple projections, up to the number of grooves in the housing.